Mix the melted butter with the crushed cookies and press into the base of a 7 inch (18 cm) loose bottomed cake tin. Put in the fridge for 15 to 20 minutes.

Meanwhile, zest the limes with a fine grater and squeeze the juice from them.

In an electric mixer, beat together the mascarpone cheese, confectioner's sugar, lime juice, zest and Cointreau.

Spread the mixture evenly over the cookie base.

Chill for at least an hour or for up to 12 hours.

Run a knife around the edge of the cake tin and carefully remove the sides.

Decorate the cheesecake with fruit or chocolate if desired.

Slice and serve.